In order to prevent loss of data, a microprocessor or data processor type load must be notified in advance of an impending power failure so that an orderly shutdown procedure can be implemented and data transferred to permanent storage so that the microprocessor may resume its tasks after power is reapplied.
In conventional line powered power supplies or off-line switchers, loss of AC line power is sensed at the AC line outlet, and a power fail signal is generated. Since the input and output circuits of the typical off-line switchers are DC voltage isolated normally by a power transformer, the same DC voltage isolation must be maintained in the alarm circuit between the AC voltage sensor and the microprocessor load. This requires the addition of a signal transformer or opto-isolator coupled to the alarm circuit.